After a lot of research, I found a way to create an applescript that will launch and renice a program. It will also take care of the whole administrator password thing for you as well. Just replace the xxxxxxxxx with your own password. I've used this with a range of programs, and all seem to work. Honestly, I can't recall why I put a 1-second delay in it; I think I just wanted to ensure that the program had launched before it reniced. I'm sure there are variations on this script. The nice thing about this is that you don't have to open Activity Monitor, find the process ID, etc. This script does all of that for you. I just save each script as an application, launch it, and everything is zippy. By the way, while I love atMonitor, it does have a reported tendency to suddenly hang your system. See the reviews for it on MacUpdate.
tell application "Safari"
activate
delay 1
end tell
tell application "System Events" to set unixID to unix id of process "Safari"
do shell script ("renice -20 " & unixID) password "xxxxxxxxx" with administrator privileges
No. Windows is not restricting your applications
If your rendering app is only single-threaded, then it can only use 100% of one CPU. So, for example,on a two-CPU machine it would show up as 50%. Windows can't make it use both CPUs because it's up to the app developer to break the task into multiple threads.
But some of the time it will have to wait while it reads input video and writes the output video... which would account for its only using 40%.
It is similar for RAM. Assuming your machine is not RAM-starved, Windows will let a process use as much RAM as it's referenced recently.
To make the compute-bound portion of video rendering go faster, you can buy a faster CPU, or buy a video card that your app can use for acceleration by doing some of the work in the GPU. Or get a different rendering app, one with better-implemented multithreading.
To make the I/O go faster, put the input, output, and temporary files on three different drives (not just drive letters. Different physical drives). Since the input files are normally the largest, and are also typically accessed in a random fashion if you're combining multiple inputs, put them on your fastest drive, an SSD if possible. The speed of the output drive won't matter much: The rate at which you can render video is a slow walk to any hard drive.
Best Answer
Realtime isn't necessarily a "no-no". It just might starve other processes out of CPU cycles. Some applications can't handle that. Its something you would have to experiment with.
High should be less of a problem. However, you still need to monitor your system to see if all the applications are behaving well.
Here is how to change the process via command line, which you can put into a shortcut:
http://support.microsoft.com/kb/191771